Reaction Level

Your unit's actual reaction score, also known as initiative, is the reaction stat multiplied by the percentage of remaining TUs. Here Max TUs refers to the maximum number of TUs that can be regenerated each turn.

As you spend time units on any actions, initiative weakens by a percentage of how many time units were spent for that action against the maximum possible time units.

If for example you have 32 of 64 Time Units and 50 Reactions, you are therefore only able to use 50% of your reaction level, or 25 out of the 50.

Let's say one step is taken, costing 4 TUs.

(32 - 4) / 64 = 0.375.

50 * 0.375 = 18.75.

Rounded down to the nearest integer, you will now only have 18 of 50 reaction to defend against reaction fire.

From this we can see that a soldier who decides to end the turn without spending any time units will often have a much higher chance to react than one that has little to no time units remaining.

This also show units with low reactions have a fighting chance at competing against an enemy with vastly superior reactions skills.

When and how is it used?

You can think of reactions as having two inherently identical but different roles: defensive and offensive.

Defensive use of reactions occurs during your turn to protect you against reaction fire.

Offensive reactions is the exact opposite, it occurs when you are not taking your turn to perform attacks of opportunities against enemy targets that have a lower initiative.

Visualise every unit on the field as having a place in an action queue based on their initiative level. All units currently making the turn will slowly drop their position in the queue as they spend their time units performing actions.

The side making the move can move any unit at any point in the queue as they please.

As soon as any unit on the waiting side jumps up in the queue above any unit on the moving side it will counter with a reaction shot as long as it meets all the conditions to perform the shot. This action costs time units, therefore instantly dropping the reacting unit's place in the queue.

This whole process keeps going until the end of the turn, and is repeated with the sides swapped. The process starts over again at the start of the next turn.

Using Reactions Defensively

Tip: Use the Mind Probe to determine if an alien is in fact out of Time Units. This can give you a good indication of whether or not the alien will be able to react should you need to shoot, capture or approach the alien to strike it with the stun rod.

The side making the current move will use their initiative levels to defend against the waiting side.

During your turn, when your soldiers are given orders that can trigger reaction fire, the unit's current reaction score is directly compared against the current reaction level of any alien that can see the unit. Naturally, if the enemy is looking the other way or there are obstructions, it will not be able to react even if it has the greater reaction level. The same applies if it doesn't have enough time units to perform an attack.

As long as your initiative is higher, you will be able to make any move you wish. The moment the enemy has a higher score, it will be able to interrupt you with an attack of opportunity - or reaction shot - if it has enough remaining time units to shoot.

In this sense, you are using your reactions to defend against enemy opportunity fire.

Using Reactions Offensively

At the end of your turn, your roles are reversed. Your units will now wait and open fire on any enemy units that are within their line of vision that have failed their initiative.

The waiting side is always on what's known as Overwatch mode, where they will attack the moving side as long as they meet several necessary conditions. This being that the moving side performs an action that brings its unit into visual range of a waiting unit. Also it must perform an act that can trigger reaction fire and the defender must have sufficient time units to perform an attack.

Player controlled units will only react with snap shots.

Working Examples

Example 1

This short example will follow two opposing units that have spotted each other. This whole exchange plays out over the alien's move.

For the sake of this example, our hero and antagonist will be called Werner Okamoto and Generic Nasty Alien Shaped Thing (GNAST). Both units will have the following stats at the end of the player's turn.

This example uses the assumption that the AI will only use snap shots in this exchange, and that both units are unable to kill each other.

Exchange Begins

Name

TUs

Reactions

Initiative >

Werner

60 / 60 (100%)

30 (30)

GNAST

25 / 50 (50%)

70 (35)

Y

Move 1 Alien fires

Werner

60 / 60 (100%)

30 (30)

Y

GNAST

5 / 50 (10%)

70 (7)

Move 2 X-COM Soldier fires

Werner

45 / 60 (75%)

30 (22)

Y

GNAST

5 / 50 (10%)

70 (7)

Move 3 X-COM Soldier fires

Werner

30 / 60 (50%)

30 (15)

Y

GNAST

5 / 50 (10%)

70 (7)

Move 4 X-COM Soldier fires

Werner

15 / 60 (25%)

30 (7)

GNAST

5 / 50 (10%)

70 (7)

Y *

Exchange Ends

* While equal, it is still the alien's move.

Example 2

Example 2 sees Werner facing a new tougher alien GNAST2, with a pistol. In this example we assume were are on the very first turn of the mission. Werner took exactly 1 step out of the Skyranger onto the ramp, and comes face to face with the enemy, who is at 100% of its time units. Werner decides to end the turn facing it.

Example 3

Example 3 has our hero facing GNAST3, a veritable clone of himself. Werner has an Autocannon this time. Through a series of convoluted explanations involving mind control, both sides wind up facing each other with 100% of their TUs in this example.